Chromium plating process



United States Patent 3,464,239 CHROMIUM PLATING PROCESS Hyman Chessin,Warren, Edgar J. Seyb, Jr., Oak Park, and Philip J. Smith, Jr., RoyalOak, Mich., assignors to M & T Chemicals Inc., New York, N.Y., acorporation of Delaware No Drawing. Filed Apr. 24, 1964, Ser. No.362,457 Int. Cl. C23b 5/50, 5/06; C23f 17/00 US. Cl. 204-29 17 ClaimsABSTRACT OF THE DISCLOSURE In accordance with certain of its aspects,the method of this invention for electrodepositing a corrosion-resistantchromium plate on the surface of a metal may comprise forming on thesurface of said metal an adherent thin film of a preferably organic,hydrogen-acceptor, film-forming composition, and electroplating achromium plate onto said metal bearing said film.

This invention relates to chromium plating. More specifically it relatesto a novel process for obtaining deposits particularly characterized bytheir high resistance to corrosion.

As is well known to those skilled in the art, decorative chromiumplating may be effected by various techniques. Although the prior artprocesses and the decorative plate produced thereby may be satisfactoryfor many purposes, platers have long appreciated that they were lessthan fully satisfactory. Specifically although it has long been desiredto quickly produce decorative plate possessing a microcrack structure,it has not heretofore been possible to accomplish this in less than12-16 minutes. Furthermore the prior art processes have not permittedattainment at low current density of thin chromium plate characterizedby its microcrack or microporous structure and resulting high resistanceto corrosion.

It is an object of this invention to permit attainment of brightdecorative chromium plate which is highly resistant to corrosion. It isanother object of this invention to obtain such plate in short platingtime and even in low current density areas. Other objects will beapparent to those skilled in the art on inspection of the descriptionwhich follows.

In accordance with certain of its aspects, the method of this inventionfor electrodepositing a corrosion-resistant chromium plate on thesurface of a metal may comprise forming on the surface of said metal anadherent thin film of a preferably organic, hydrogen-acceptor,film-forming composition, and electroplating a chromium plate onto saidmetal bearing said film.

The basis metals which may be treated by the process of this inventionmay include metals such as iron, steel, brass, bronze, copper, zinc,aluminum, magnesium, nickel, etc. The preferred basis metal which may beplated may be steel, zinc, or brass, and preferably steel, zinc, orbrass which has been plated with a plate of nickel, typically precededby a first plate of copper.

The basis metal, preferably bearing a nickel plate, may be cleaned as bycathodically treating in an alkaline cleaner and rinsing in water priorto further treatment. The sotreatment metal may be dried or it may befurther treated as 1s.

-In practice of certain aspects of this invention, the sotreated metalmay be treated with an organic, hydrogen- 3,464,899 Patented Sept. 2,1969 acceptor, film-forming composition. It has unexpectedly been foundthat use of preferably organic, hydrogen-acceptor, film-formingcompositions permits attainment of the novel results hereinafter setforth. These compositions may be characterized by their ability toreadily accept, i.e., to react with, hydrogen (particularly in the formof nascent hydrogen) in the presence of activated metal catalyst. Theymay be characteristically considered as hydrogen overvoltage poisons,i.e., when they are added to a system evolving hydrogen at a cathode, ahigher potential is necessary to continue the hydrogen evolution at thesame rate. They may be particularly characterized by their ability toraise the hydrogen overvoltage of a standard hydrogen electrodetypically by at least about 0.20 volt. This may be observed byelectrolyzing a solution which is one molar in acetic acid and one molarin sodium acetate at a current of 30 milliamps over an 8.25 cm. steelcathode. It will be found that the hydrogen overvoltage may be raised byat least about 020- volt when the composition is added in amount ofsaturation up to 0.0232 molar, q.v. Duwell, E.J., Jour. Electrochem.Soc., vol. 109, pp. 1013-1017 (1962).

The compositions used in practice of the process of this invention maybe found to possess a high degree of adherence to and/ or adsorption onthe basis metal cathode, particularly during the initial period (i.e.,the first few seconds) of chromium plating.

Preferably the organic composition may be one containing at least onepoint of non-aromatic unsaturation e.g., nonaromatic double or triplebonds including, e.g., -CEC, C=C-, -CEN, --C=N, -C=S, -N=N--, and theC=O group as in quinone. A typical composition may include more than onepoint of unsaturation, e.g., it may contain at least two -C=C- bonds.

The preferred composition may be formed from at least one materialselected from the polymer-forming group consisting of acrylate,acrylonitrile, butadiene and styrene. These compositions may be used inthe form of monomers or preferably in the form of polymers. Typicalcompositions may be formed from inertly substituted materials includingisoprene, i.e., 2-methylbutadiene, etc. Preferred compositions mayinclude polymers and derivatives of acrylonitrile such aspolyacrylonitrile and acrylic rubber, polymers of butadiene'includingnatural rubber (i.e., poly-Z methylbutadiene), and polymers of styreneincluding polystyrene. Copolymers such as butadienestyrene,butadiene-acrylonitrile, or acrylonitrile-butadienestyrene may be used.Modified polymers including modi fied butadieue-acrylonitrile may beemployed wherein the acrylouitrile residue in the molecule may have beenhydrolyzed to the acrylic acid or salt, i.e., the --CN groups may havebeen hydrolyzed to the -COOH group. A preferred composition may includemethyl acrylate or ethyl styreneacrylonitrile together with a fatty acidemulsifier having a pH of 9.5 and an average particle size of about 400Angstrom units (such as that sold by B. F. Goodrich under the trademarkHycar 1577);

(b) A latex containing a carboxylated copolymer ofacrylonitrile-butadienestyrene together with alkyl aryl sulfonateanionic detergent having a pH of 8.0 and an average particle size ofabout 1200 Angstrom units (such as that sold by B. F. Goodrich under thetrademark Hycar 1570-X20);

(c) A butadiene-styrene copolymer containing a fatty acid emulsifierhaving a pH of 10.0 and having an average particle size of 600 Angstromunits (such as that sold under the trademark Naugatex 2006 by NaugatuckChemical 00.);

(d) A butadiene-acrylonitrile-carboxylic modifier latex (i.e., a polymerwherein the CN groups of the nitrile have been hydrolyzed to COOHgroups) having an anionic emulsifier having a pH of 8.5 and an averageparticle size of about 1800 Angstrom units (such as that sold under thetrademark Chemigum 520 by Good year Co.);

(e) A natural rubber latex having a pH of 10 and an average particlesize of about 100200 Angstrom units; (f) A 0.5% solution of naturalrubber in benzene.

Other illustrative compositions which may be used may include:

(g) A solution of 0.34 g. of Carters Rubber Cement in 100 ml. ofpetroleum naphtha (Carters Thinner sold by The Carters Ink Co.);

(h) A mixture of 45 ml. of water and 1 ml. of a commercial latex paint,Glidden Spred brand, Satin, garnet maroon 3438 containing syntheticrubber in a vehicle;

(i) A latex of styrene-butadiene such as that sold under the trademarkSBR-2000 by B. F. Goodrich C0.

The preferred form in which these compositions may be employed may be inthe form of a latex in aqueous medium having a concentration of0.001%60%, say 0.05%. The preferred medium may be natural rubber latexwhich is an aqueous dispersion of 3% of rubber, i.e., predominantlyZ-methylbutadiene.

Other organic, hydrogen-acceptor, film-forming, monomeric compositionswhich may be employed may include acetylenic compounds, i.e., compoundscontaining a CEC group such as phenyl acetylene; heptyne-l;heptadiyne-1,7; hexyne-3; 2,S-dimethyl-3-hexyne-2,5-diol; propargylmalondiamide; hexyn-5-ol; hexyne-Z; etc.

Other suitable hydrogen-acceptor, film-forming monomeric compositionswhich may be employed may include sulfur compounds including: sulfur;carbon disulfide; dicyclopentamethylene thiuram monosulfide;dicyclopentamethylene thiuram disulfide; dicyclopentamethylene thiuramtetrasulfide; 2,5 dimercapto-l,3,4-thiadiazole;phenyl-Z-mercaptobenzimidazol; piperidine pentamethylenedithiocarbamate; dibenzothiazyl dimethyl thiol urea; o-mercaptobenzoicacid; diethyldithiocarbamic acid (Na salt); tetramethyl thiuramdisulfide; tetrabutyl thiuram disulfide; o-mercaptosuccinnic acid; 2mercaptothiazoline; 2 mercaptoethanol; bis(2-hydroxyethyl)dithiocarbamicacid (K salt); bis(p-nitrophenyl)disulfide; and 2,2'-dithiobenzoic acid(K salt).

Other illustrative hydrogen-acceptor, film-forming compositions whichmay be employed may include: 2- nitrobenzene arsonic acid; arsenousacid; p-(phenyl selenyl)aniline; ethylene dicyclohexanol; hippuric acid;perthiocyanic acid (as Na salt); antimony oxide Sb O and arsenic oxideAS203.

When the hydrogen-acceptor, monomeric, film-forming composition isemployed in practice of this invention, it may preferably be employed inappropriate solution, typically of 0.005% in concentration up tosaturation. Sulfur, if used, may be dissolved in isopropanol. Othercompositions may be dissolved, suspended, etc., in appropriate mediumtypically water, alcohols such as methanol, ethanol, propanol, etc.,hydrocarbons such as benzene, xylene, toluene, etc.

The composition may be deposited onto the metal by cathodic treatment.For example, arsenous acid, arsenic oxide, antimony oxide, etc., may bedeposited by cathodizing the metal in a solution (e.g., of sulfuricacid) containing these materials.

It is a feature of this invention that the novel results attainablethereby may be enhanced, particularly when the organic,hydrogen-acceptor, film-forming composition is a latex of thepolymer-forming group supra consisting of acrylate, acrylonitrile,butadiene, and styrene, by the use in the body of composition of alyophilic protective colloid. Typical of such colloids may be gelatin,agar, gum tragacanth, karaya gum, ethyl cellulose, alginates, methylether of cellulose, hydroxyethyl cellulose, carboxymethyl cellulose,pectin, etc. They may be added to the latex in amount of 0.005%10% ormore.

A typical composition may include methyl ether of cellulose (Methocelbrand) in amount of 4 g./l. in an aqueous latex containing 7.5% ethylacrylate polymer.

Treatment of the basis metal (including plate metal previously depositedthereon, and typically steel bearing a first plate of copper and asecond plate of nickel) with the composition in accordance with thisinvention may include dipping, spraying, electrolytically treating,swabbing, electrophoretically depositing, or dabbing the compositiononto the basis metal. Preferably this may be effected by dipping themetal in a body of the latex. Preferably the metal surface may bemaintained in contact with the composition at 10 C.50 0, say 20 C. for1-60, say 5 seconds or longer and then withdrawn from contact. When themetal is introduced dry, the time may be very short. When the metal isintroduced with a wet surface, the time may be longer and preferably therate of introduction to the body should be less than about 10centimeters per minute.

Preferably the metal surfaces bearing the films formed from thecomposition in latex form may be rinsed with water. Rinsing may removethe excess of composition and leave a film on the surface. These filmsmay be dried to a uniform layer (that may be a monomolecular film) priorto rinsing.

It is a particular feature of the compositions noted that when appliedto the metal surface, they may form a film. This film may bemore-or-less continuous, i.e., substantially uniformly distributed onthe entire surface of the metal. In case of certain compositions, e.g.,sulfur, the sulfur may be uniformly but discontinuously distributed overthe entire surface. In case of other compositions, e.g., a naturalrubber latex, the rubber may form a very thin and substantiallycontinuous film. Commonly such a film may have a thickness of -2000, say400 Angstrom units and typically it may be a monomolecular film. Thefilms, which may be invisible to the naked eye, remaining on the metalsurface may be characterized by their high degree of adsorption andtheir adherence.

The so-developed film may preferably be cathodically treated, i.e.,maintained as cathode for 1-60 seconds, say 30 seconds preferably inacid solution, typically 0.5%- 10%, say 2% by weight aqueous solutionsof sulfuric acid. Preferably the cathode current density may be 0.5-8a.s.d., say 4 a.s.d.

The metal surface bearing the film may then be placed in a chromium bathand chromium plating may be initiated. As plating proceeds onto thecathode surface bearing the film, the film may gradually during 3-180seconds, typically 5-10 seconds be dissolved, dissipated, or etched offthe cathode. The dissolution of the film may initiate at selected spotsmore-or-less uniformly distributed over the surface of the cathode atwhich the initial deposit of chromium may form. Substantially the entirefilm may be removed from the metal surface by the action of the evolvinghydrogen at the cathode, and reaction with the chromium platingsolution. It is a particular feature of these films that they arecontrollably removed under the peculiar'conditions prevailing at thecathode in a chromium plating bath, i.e., in the presence of largeamounts of nascent hydrogen and of highly oxidizing chromic acid.- 7

Chromium plating may be effected at temperatures of 30 C.-60 C., say 50C., and 5-50, say'30 a.s.d. for 0.5-15, say 1-9 minutes from a bathcontaining 100-500 g./l., say 250 g./l. of chromic acid and 1-5 g./l.,say 2.5 g./l. of sulfate ion, typically derived from sodium sulfate.Other components including other chromium plating catalysts, e.g.,fluoride or silicofiuoride, self-regulating compositions, fumesuppressants, etc., may be present.

The chromium plate prepared by the process of this invention may beparticularly characterized by its bright decorative appearance, its highcorrosion resistance, and by its microcracked or microporous structurein a thin layer. When this plate is formed in decorative thickness, itmay be found that it is characterized by unexpectedly superiorproperties.

It is a particular characteristic of the novel deposit that it does not,in contrast to prior art plate deposited directly on the basis metal,acquire the stress pattern of the basis metal. Rather it acquires a lowstress which is characteristic of the novel deposited plate.

The novel deposit is unexpectedly characterized by excellent corrosionresistance at very low thickness, typically as'low as 0.1 micron andsimultaneously by a microcrack pattern attained at thickness as low as0.25-0.75 micron, say 0.3 micron, not heretofore attainable. Theunexpected ability of this novel process to permit attainment of thenoted crack pattern at such low thickness is particularly outstanding.The plate is normally characterized by presence of fine cracks,typically in amount of 20- 800 or higher, say 400 per centimeter. Evenat thicknesses below which a crack pattern is attained, the chromiumpossesses an unusual microporosity which during corrosion testingdevelops into a disconnected microcrack pattern in the forms of stars orcrowfeet.

This novel product when tested for corrosion as conducted under ASTMSpecification B368-61T (CASS- Test) and B-117-62 (neutral salt spray)may be found to be substantially superior to a standard chromium plateof the same thickness when prepared by conventional methods.

The novel chromium plate of this invention may be characterized by itsunique combination of low thickness, as low as 0.1-0.75 micron, commonly0.3 micron, and its fine crack pattern in thickness as low as 0.25micron. As the plate is deposited and goes from to 0.25 micron, thechromium deposit may be characterized by a plurality of holes eachtypically of diameter of 0.01-0.2 micron, say q 04 micron and spaced ata distance from each other of 50-200, say 100 microns. Typically'als'othe plate may include (as demonstrated by testing in the CASS testfollowed by copper plating in the standard Dupernell test, to revealcrack structure) a plurality of nonconnected, nonintersecting, very fineattenuated crack lines, each 1-4 microns, say 2 microns long andradiating from the said holes. Typically each hole may have less thanabout 5 crack lines radiating therefrom; most commonly 3 cracks may bepresent in the form of a star or a crowfoot. The number of holes presentin this form may be 1000-4000, say 2000 per square centimeter.

As the thickness of the plate increases to the preferred thickness of0.25-0.75 micron or more, the crack pattern which develops may include aplurality of intersecting microcracks, typically in amount of 20-800 percentimeter or higher, say 400 per centimeter.

The novel chromium plate of this invention, which may be particularlycharacterized'by its crack pattern of typically 20-800 cracks percentimeter in decorative thickness of 0.25-0.75 micron, may be found topossess unexpectedly superior corrosion-resistant properties in thesedecorative thicknesses. When tested as by the CASS test 6 supra, it willbe found that at a decorative thickness of, e.g., 0.25 micron, the novelplate is completely satisfactory while a control plate of the samethickness produced by prior art techniques is totally unsatisfactorywith respect both to appearance of the chromium plate and attack of thebasis metal.

The novel chromium plate of this invention, which also may be unexpectedcharacterized by microporosity in thickness as low as 0.1 micron may befound to possess unexpectedly superior corrosion resistant properties inthese decorative thicknesses. When tested as by the CASS test supra, itwill be found that at a decorative thickness of, e.g., 0.125 micron, thenovel plate is satisfactory while a control plate of the same thicknessproduced by prior art techniques is totally unsatisfactory with respectto appearance of the chromium plate and attack of the basis metal.

It appears that the unexpected properties of this novel plate maypossibly be related to the hydride content of the initial layer ofchromium deposit. According to the theory of Cloyd Snavely (Trans.Electrochem. Soc., vol. 92, 537-577, 1947), it appears that chromiumhydride may be codeposited with chromium during electrodeposition, andthe stress in the chromium deposit may arise from the conversion of thishydride to chromium metal and the simultaneous release of hydrogen.Cracking in chromium deposits may appear at this time because the newlyformed chromium lattice is smaller than the lattice of the depositedchromium hydride. The novel process of this invention appears to permitincrease in the hydrogen overvoltage during the initial stages ofdeposition so that a larger proportion of hydride is formed. Thus, whenconversion to chromium metal occurs, the initial stress which causescracking may occur sooner, i.e., when the deposited plate is thinner.

Practice of the process of this invention may be more apparent frominspection of the following examples wherein all parts are parts byweight unless otherwise specified.

In one standard example, a brass panel 75 mm. high x mm. wide which hadbeen bright-nickel plated was employed. It was subjected to thefollowing steps:

(a) Cold Water rinse.

(b) Dip into 2% sulfuric acid by volume.

(c) Cold water rinse.

((1) Hot water wash.

(e) Blow dry.

(f) Treat the lower 25 mm. of the plate as set forth in column 2 ofTable I. This is the experimental portion. The 25 mm. immediately abovethis experimental portion is the control portion.

(g) Cathodize entire panel in a Hull cell containing 5% sulfuric acid byvolume for 30 seconds at 5 amperes.

(h) Cold water rinse.

(i) Chromium plate in a standard cell by making immediate contact andquickly raising the current to 7.5 amperes for three minutes at 42 C.-43C. in a bath typically containing:

G./l. cro 200 so.,= 1.5 SiF 2.0

In each example, the nickel plated basis metal was treated in mannergenerally similar to that set forth in the standard example except thatin the step there designated as step (f), the lower portion of the panelwas treated in the manner set forth in Table I.

' After the plating had been effected from a bath such as that typifiedby the standard example, the number of cracks per centimeter wasdetermined for the upper or control portion of the plate and for thelower or experimental portion of the plate. The number of cracks percentimeter for the control was determined by measuring across a linegenerally perpendicular to the plurality of parallel gross cracks whichcharacterized the substrate on which the control is deposited. These Y-or grosscracks reflect the surface structure of the basis metal ratherthan that of the microcrack pattern; and in fact no microcrack patternwas observed in the control areas. The number of cracks for theexperimental was determined by measuring along a randomly positionedline.

TAB LE I Example Treatment Cracks/cm.

C ontrol Experimental 1 Swab with a solution made by thinning 1 ml. ofrubber cement (Carters Rubber Cement containing rubber in petroleumnaphtha) with 30 ml. of petroleum ether.

2 Swab with a solution made by adding 1 ml. of latex paint (GliddenSpred Satin, garnet maroon 343B) containing synthetic rubber emulsion invehicle, to 45 ml. of water.

3 Swab with natural rubber latex (B. F. Goodrich 00.).

4 Spray with a synthetic polymer latex (B. F. Goodrich Co. Hycar 1577)formed from styrene-butadiene-ac y onitrile;

5 Dip into a solution of 5 ml. of

synthetic polymer latex (B. F. Goodrich Co. SEE-2000 latex) formed fromstyrene-butadiene, ml. distilled water and ml. acetone.

6 Dip into solution formed by adding 90 ml. distilled water to 10 ml. ofa synthetic polymeric acrylonitrile latex (B. F.

Goodrich Co. Hycar 1552 meduim acrylonitrile type latex).

7 Dip for 60 seconds into solution formed by mixing equal parts ofdistilled water and a synthetic polymeric (B. F. Goodrich Co. Hycar1570-X-) carboxylated acrylonitrile latex.

8 Dip for 30 seconds into a solution formed by mixing equal parts ofdistilled water and a synthetic polymeric (Goodyear Co. Chemigum Latex520) carboxylic modified butadieneacrylonitrile latex.

9 Spray with a solution formed from 10 ml. ethynylbenzene, 60 ml.isopropanol, 30 ml. distilled water.

10 Swab with a solution made by boiling with excess isopropanol, 5 g. ofa plastic material formed by polymerizing trimethyl dihydroquinolinewith 1% by weight conc. sulfuric acid.

11 Swab with a solution made by boiling for 4 minutes an aqueoussolution containing 14.8 g./l. of sodium hydroxide and 5.8 g. ll. of asynthetic (General Aniline Gantrez AN-3132) ethyl half ester of acopolymer of methyl vinyl ether and maleic anhydride (Gantrez AN 119).

12 Swab with a solution in isopropanol of 5 g./l. of a synthetic(General Aniline Gantrez AN- 3391) methyl halt ester of a copolyrner ofmethyl vinyl ether and maleic anhydride (Gantrez AN-139).

13 Swab with an aqueous saturated solution of hippuric acid, -CO-NH-CHz-COOH, let dry.

14 Swab with hot solution containing 0. 15 g. of dicyclopentamethylenethiuram monosulfide in 50 ml. isopropanol.

15 Swab with hot solution containing 0. 15 g. of dicyclopentamethylenethiuram disuliide in 50 ml. isopropanol.

16 Immerse into solution containing 100 ml. distilled water, 2 ml.sulfuric acid, and 0.1 g. p- (phenyl selenyl) aniline lrP-CBH5NH2.

17 Swab with warm isopropauol solution oi product prepared by heating at105 C. for 7.5 hours, 2.2 g. of dicyclopeutamethylene thiuranidisullldc.

TAB LE I (Continued) Cracks/cm.

Treatment Control Experimental Swab with solution prepared 20-30 bysaturating boiling isopro- 19 Swab with solution prepared by 4-8 120dissolving in 25 m1. of isopropanol0.1 g. of 2, 5-dimercapto- 1, 3,4-thiadiazole, let dry.

20 In a Bull cell, cathodize the lower half of the panel for 30 secondsat 5 amperes in an aqueous solution containing excess arsenous acid andhydrochloric acid to lower the pH to 1.2.

21 In a Hull cell, cathodize the lower half of the panel in an aqueous2% solution of sulfuric acid saturated with arsenous acid AS203.

22 u In a Hull cell, cathodize the lower half of the panel in an aqueous2% solution of sulfuric acid saturated with antimony oxide SbzOg.

23 Swab squalene onto the lower portion of the panel.

24 Swab hydroxypropyl methacrylate monomer onto the bottom of the panel.

25 Swab allyl propionate onto the lower portion of the panel.

26 Swab styrene onto the lower portion of the panel.

27 Dip the lower portion of the panel into a solution containing 25% byvolume of a synthetic polymer latex (B. F. Goodrich C0. Hycar 1577)formed from styrene-butadiene-acrylonitrile and 75% by volume of asolution of 4. 1 g.ll. agar-agar in water adjusted to pH 10 withammonium hydroxide.

28 Dip the lower portion of the panel into a solution containing 15% byvolume of a synthetic polymer latex (B. F. Goodrich Go. Hycar 1577)formed irorn styrene-butadiene-acrylonitrile and by volume of an aqueoussolution of 4.7 g./l. of karaya gum.

29 Dip the lower portion of the panel into a solution containing 15% byvolume of a synthetic polymer latex (B. F. Goodrich Co. Hycar 1577)formed from styrene-butadiene-acrylonitrile and 85% by volume of anaqueous solution of 4.7 g./l. gum tragacanth. 30 Dip the lower portionof the panel i into a solution containing 14 ml.

of a synthetic polymer latex (B. F. Goodrich Co. Hycar (1577) formedfrom styrenebutadieneacrylonitrile, 43 ml. of a solution of 25 g./l. ofgelatin which was adjusted to a pH of 9. 5 by addition of ammoniumhydroxide, to which distilled water was added to make min In anotherexample, No. 31, a nickel plated panel was treated in the standardmanner supra'through steps (e). For step (f), the lower 25 mm. of thepanel was dipped into-a 20% by volume solution of an acrylate (B. F.Goodrich Hycar 2600 X94). Then the panel was water rinsed andplated 'at48 C. in the standard Hull cell at 10 amperes for seven minutes in thefollowing chromium plating solution: CrO 250 g./l. and S0 2.5 g./l.There was no cracking in the untreated control area, while the crackcount on the test area was 50 cracks per centimeten.

Inspection of the results of the typical illustrative exampleshereinbefore set forth reveals that, inall cases, treatment of the basismetal prior to plating in accordance with the novel process of thisinvention permits attainment of a crack pattern characterizedby the.presence of substantially more cracks per centimeter than that of thecontrol. In one case, Example 22, the number of cracks was increased bya factor of as much as,l00 times. As will be apparent, an increase by afactor of 5-10 is common.

.9 More significant however is the fact that practice of the novelprocess of the instant invention gives a microcrack pattern whichpermits attainment of a degree of corrosion resistance heretoforeunattainable in decorative plate of such low thickness.

In order to illustrate the high degree of superiority of the novel plateproduced by the process of this invention, comparative corrosion testswere run as follows:

CORROSION TESTS In this comparative test, 100 mm. x 150 mm. steel panelswere vapor degreased, anodically. cleaned, scrubbed, water rinsed,dipped in 6% aqueous solution of hydrochloric acid, water rinsed, andnickel plated for 80 minutes to produce thereon a bright nickel platehaving a thickness of 1.0 mil. The panel was then water rinsed, dippedin 2% aqueous sulfuric acid, water rinsed, and dipped into a solutioncontaining 50% by volume of the synthetic polymer latex (B. F. GoodrichCo. Hycar 1577) formed from styrene-butadiene-acrylonitrile and 50%water. The experimental panel was removed from this bath and waterrinsed. A similar control panel was treated in identical manner in allrespects except that it was not dipped into thestyrene-butadiene-acrylonitrile latex solution.

Samples of the control panel and the experimental panel were eachindependently chromium plated in a first chromium plating bathcontaining 190 g./l. CrO 1.03 g./l. SO and 1.64 g./l. SiF (frompotassium silicofluoride) or in a second bath containing 240 g./l. CrO0.9 g./l. SO and 1.9 g./l. SiF Plating in each case was effected at 43C. and 15 a.s.d. Selected panels, control and experimental, which hadbeen each plated for 1 minute, 2 minutes, and 4 minutes to producerespectively plates of 0.25 micron, 0.50 micron, and 1.0 micronthickness were each tested by the 24-hour CASS test in standard mannerto determine-corrosion. The results are measured in standard manner on a-10 scale wherein 10 is perfect (no corrosion) and 0 is totallyunsatisfactory. A value of 7 or less is considered commerciallyunacceptable and a value of 9 or 10 is acceptable. The results of thetests are as follows:

TABLE II.-FIRST BATH Rating Thickness Experimental Control TABLETIL-SECOND BATH Rating Thickness Experimental Control Selected panelsmaybe found to have crack patterns, expressed in cracks per centimeter, asfollows, these correspondingto Table III supra. z

" TABLE IV.SECOND BATH Thickness Experimental Control 10 16 hours in theCASS test and rated for corrosion as before. The results are given inTable V.

. TABLE V Thickness Experimental Control After the accelerated corrosiontest, the test panels did not show a continuous connected crack patternbut rather a fine porosity described previously in which some of thepores had developed 2-5 radiating cracks in the form of stars orcrowfeet. These average approximately 2000 per square centimeter.

Two irregularly shaped steel parts (license plate brackets) were platedin the same fashion as the panels described above. The test parts weredipped in the latex solution while control parts were not. Both wereplated in the second bath supra at amperes for nine minutes. Thechromium thickness in the recess was approximately 0.1 micron and 2microns at the protuberances. The crack density was found to be about750 cracks per centimeter on the protuberances of the experimental part.These parts were exposed in the CASS test for 76 hours. Particular notewas made of the recesses where the chromium was thin. With theexperimental parts, one part showed no corrosion at all (rating of '9)in the recesses. With the control parts, the rating was 0 in therecesses for both parts. All parts rated 10 where the chromium wasthick.

Another set of parts were treated as before and plated in the secondbath supra but with a higher current of 300 amperes for the nineminutes. The thickness range was from about 0.25 microns in the recessesto about 5 microns on the protuberances. The crack density was found tobe about 1000 cracks per centimeter on the protuberances of theexperimental part. The parts showed no corrosion (rating of 10) both inthe recesses and where the thickness was greater, after 76 hours CASStesting.

From Tables II and III, it will be apparent that the corrosionresistance of the novel plate is superior to that of the control in thedecorative thicknesses deposited. For example a thickness of 0.25 micronwas found to be totally satisfactory when achieved by the process of thepresent invention whereas one prepared by the control process which wastwice as thick, 0.50 micron, was found to be unsatisfactory from thepoint of view of corrosion.

Furthermore, it is apparent from Table IV that the novel product ischaracterized by the unique combination of microcrack pattern at theunusually low decorative thickness of less than about 1 micron. It isalso apparent from Table V, and the example using parts, that at verylow chromium thicknesses where continuous chromium cracking does notoccur, outstanding corrosion protection is still attained. It will befurther apparent that the novel plate of this invention may be depositedin greater thickness in the typical decorative range up to about 5microns or more as illustrated by the parts plated at the greaterchromium thicknesses, but that the advantages herein disclosed arepeculiarly outstanding at lower thicknesses.

Although this invention has been described by reference to certainspecific examples, it will be apparent to those skilled in the art thatvarious changes may be made therein which fall within the scope of thisinvention.

We claim:

1. A novel chromium plating process for plating onto a metal adecorativechromium plate characterized by its high resistance tocorrosion in typical decorative thicknesses of 0.1-5 microns havingsubstantially increased crack density and a corrosion rating by the CASStest of at least 8, which comprises forming on the surface of said metalan adherent thin film of a hydrogen-acceptor, film-forming composition,and electroplating from an aqueous bath a chromium plate onto said metalbearing said film said hydrogen-acceptor being capable of raising 11 thehydrogen overvoltage of a standard hydrogen electrode by at least 0.20volt.

2. A novel chromium plating process as claimed in claim 1 for platingonto a metal a decorative chromium plate characterized by its highresistance to corrosion in typical decorative thicknesses of 0.1-microns wherein said hydrogen-acceptor, film-forming composition is anorganic composition containing at least one point of nonaromaticunsaturation.

3. A novel chromium plating process as claimed in claim 1 for platingonto a metal a decorative chromium plate characterized 'by its highresistance to corrosion in typical decorative thicknesses of 0.1-5microns wherein said hydrogen-acceptor, film-forming composition isformed from at least one material selected from the group consisting ofacrylate, acrylonitrile, butadiene, and styrene.

4. A novel chromium plating process as claimed in claim 1 for platingonto a metal a decorative chromium plate characterized by its highresistance to corrosion in typical decorative thicknesses of 0.1-5microns wherein said hydrogen-acceptor, film-forming composition isformed from ethyl acrylate.

5. A novel chromium plating process as claimed in claim 1 for platingonto a metal a decorative chromium plate characterized by its highresistance to corrosion in typical decorative thicknesses of 0.1-5microns wherein said hydrogen-acceptor, film-forming composition isformed from natural rubber.

6. A novel chromium plating process as claimed in claim 1 for platingonto a metal a decorative chromium plate characterized by its highresistance to corrosion in typical decorative thicknesses of 0.1-5microns wherein said hydrogen-acceptor, film-forming composition is alatex.

7. A novel chromium plating process as claimed in claim 1 for platingonto a metal a decorative chromium plate characterized by its highresistance to corrosion in typical decorative thicknesses of 0.1-5microns wherein said hydrogen-acceptor, film-forming compositionincludes a latex of natural rubber.

8. A novel chromium plating process as claimed in claim 1 for platingonto a metal a decorative chromium plate characterized by its highresistance to corrosion in typical decorative thicknesses of 0.1-5microns wherein said hydrogen-acceptor, film-forming composition is anacrylonitrile-butadiene-styrene polymer.

9. A novel chromium plating process as claimed in claim 1 for platingonto a metal a decorative chromium plate characterized by its highresistance to corrosion in typical decorative thicknesses of 0.1-5microns wherein said hydrogen-acceptor, film-forming composition is anacetylenic compound.

10. A novel chromium plating process as claimed in claim 1 for platingonto a metal a decorative chromium plate characterized by its highresistance to corrosion in typical decorative thicknesses of 0.1-5microns wherein said hydrogen-acceptor, film-forming composition is anacetylenic compound selected from the group consisting of heptyne-l;heptadiyne-1,7; hexyne-B; 2,5-dimethyl-3- hexyne-2,5-diol; propargylmalondiamide; hexyne-S-ol; and hexyne-Z.

11. A novel chromium plating process as claimed in claim 1' for platingonto a metal a decorative chromium plate characterized by its highresistance to corrosion in typical decorative thicknesses of 0.1-5microns wherein said hydrogen-acceptor, film-forming composition is asulfur-containing composition.

12. A novel chromium plating process as claimed in claim 1 for platingonto a metal a decorative chromium 7 plate characterized by its highresistance to corrosion in .typical decorative thicknesses of 0.1-5microns wherein said hydrogen-acceptor, film-forming composition isselected from the group consisting of sulfurt'carbon disulfide;dicyclopentamethylene thiuram monosulfide; dicyclopentamethylene thiuramdisulfide; dicyclopentamethylene thiuram tetrasulfide;2,5-dimercapto-1,3,4-thiadiazole; phenyl-Z-mercaptobenzimidazol;piperidine pentamethylene dithiocarbamate; dibenzothiazyl dimethyl thiolurea; o-mercaptobenzoic acid; diethyldithiocarbamic acid; tetramethylthiuram disulfide; tetrabutyl thiuram, disulfide; o-mercaptosuccinicacid; 2-mercaptothiazoline; Z-mercaptoethanol;his(2-hydroxyethyl)dithiocarbamic acid; bis(pnitrophenyl)disulfide; and2,2'-dithiobenzoic acid.

13. A novel chromium plating process as claimed in claim 1 for platingonto a metal a decorative chromium plate characterized by its highresistance to corrosion in typical decorative thicknesses of 0.1-5microns wherein said hydrogen-acceptor, film-forming composition isselected from the group consisting of Z-nitrobenzene arsonic acid;arsenous acid; p-(phenyl selenyl)aniline; ethylene dicyclohexanol;hippuric acid; perthiocyanic acid; antimony oxide Sb O and arsenic oxideAs O 14. A novel chromium plating process as claimed in claim 1 forplating onto a metal a decorative chromium plate characterized by itshigh resistance to corrosion in typical decorative thicknesses of 0.1-5microns wherein said adherent thin film of composition is formedin thepresence of and contains a lyophilic protective colloid.

15. A novel chmomium plating process as claimed in claim 1 for platingonto a metal a decorative chromium plate characterized by its highresistance to corrosion in typical decorative thicknesses of 0.1-5microns wherein said lyophilic protective colloid is selected from thegroup consisting of gelatin, agar, gum tragacanth, karaya gum, ethylcellulose, alginates, methyl ether of cellulose, hydroxyethyl cellulose,carboxymethyl cellulose, and pectin.

16. A novel chromium plating process for plating onto a metal adecorative chromium plate characterized by its high resistance tocorrosionin typical decorative thicknesses of 0.1-5 microns havingsubstantially increased crack density and a corrosion rating by the CASStest of at least 8, which comprises forming on the surface of said metalan adherent film of a hydrogen-acceptor, filmforming composition, dryingsaid film thereby forming a dry adherent thin film of hydrogen-acceptor,film-forming composition, and electroplating from an aqueous bath achromium plate onto, said metal bearing saidfilm, said hydrogen-acceptorbeing capable of raising the hydrogen over-voltage of a standardhydrogen electrode by at least 0.20 volt. r

17. A novel chromium plating process as claimed in claiml for platingonto a metal a decorative chromium plate characterized by its highresistance to corrosion in typical decorative thicknesses of 0.1-5microns including the step of cathodically treating said film prior tochromium plating.

References Cited UNITED STATES PATENTS "2,846,380 8/1958 Brown 204-513,041,257 6/1962 Cope et al 204 51 1,283,973 11/1918 Thum 6121 204-291,839,905 1/1932 Tainton 1 204-51 2,899,367 8/1959. Veeder 204-323,129,149 4/1964 Johnson 204-29 3,175,964 3/1965 Watanabe 1 al. 204-37FOREIGN PATENTS 1,105,683 4/1961 Germany.

. 1.117.96 il G rm y JOHN H. MACK, Primary Examiner W: B. VAN S ISE,Assistant Examiner US. Cl. X.R.' 204---38, 51 l

